INTRODUCTION

A photographic collage depicting the diversity and evolution of life on Earth
through the last 600 million years. The oldest fossils are at the bottom and
youngest at the top. The size of each time interval is proportional to its
duration.

We study our Earth for many reasons: to find water to drink or oil to run our
cars or coal to heat our homes, to know where to expect earthquakes or landslides
or floods, and to try to understand our natural surroundings. Earth is constantly
changing--nothing on its surface is truly permanent. Rocks that are now on top of
a mountain may once have been at the bottom of the sea. Thus, to understand the
world we live on, we must add the dimension of time. We must study Earth's
history.

When we talk about recorded history, time is measured in years, centuries, and
tens of centuries. When we talk about Earth history, time is measured in millions
and billions of years.

Time is an everyday part of our lives. We keep track of time with a marvelous
invention, the calendar, which is based on the movements of Earth in space. One
spin of Earth on its axis is a day, and one trip around the Sun is a year. The
modern calendar is a great achievement, developed over many thousands of years as
theory and technology improved.

People who study Earth's history also use a type of calendar, called the geologic
time scale. It looks very different from the familiar calendar. In some ways, it
is more like a book, and the rocks are its pages. Some of the pages are torn or
missing, and the pages are not numbered, but geology gives us the tools to help
us read this book.

This page is URL: https://pubs.usgs.gov/gip/fossils/intro.html
Last updated 26 June 1997 (krw)
Maintained by John Watson